I'd put it the other way around: Bad Grammar is a courtesy. I run a startup that's small, but busy. I get a high frequency stream of inbound questions, notifications and asks to make decisions by my team and customers. If I don't respond or decide quickly I become a bottleneck. Likewise, if I wait, things pile up. So, rather than keep everyone waiting for me, I make a point of pulling my phone out as soon as I get a message and provide an answer straight away as much as possible. These answers are brief and to the point. And they are laden with shitty grammar. But they are almost instant and that feels better than a well formulated essay two hours later.

Having said that, I started using Gmail's "polish" feature to turn "yes" into "That sounds great, let's go ahead with it" or some such corporatism. Not sure if that's much better...